In the drilling and production of wells many instruments such as gravity meters are lowered into the well bore. Such instruments function most effectively if their reference axis can be positioned substantially vertically. In the drilling of wells, the well bores have varying inclinations, sometimes intentionally in directional drilling but most often just as the result of normal drilling operations in which the drill bit does not proceed vertically.
The use of instruments within a well bore dictates that the instrument be sufficiently small so that it may be lowered into the well bore. Thus the controlling dimension for the instrument is the diameter of the well bore. This restricts the size of the housing for the instrument and also the degree of tilt which may be achieved to correct for well bore inclination. The design of well instruments is not limited in longitudinal length because an increase in length along a well bore does not require that the bore be enlarged as increasing the diameter would require.